"What I care about is "what will be the outcome if we do this?". Will it be good for women, good for men, good for trans people, bad for women, bad for humanity?"
This is exactly how I think about it, too: What will be the consequences if we say "Any man who puts on makeup and says 'I feel like a woman today'?
"If you send a message out to society that women are more more weak and vulnerable than is proportionate, society will believe it. And women will too. It will set us back so many years."
It is not a made up belief that women are physically weaker than man AND are disadvantaged in some ways. That is why there is such a thing as Women's Sports separate from men's. That is why some schools give girls' scholarships. That is why we talk about women's issues like forced marriages, FGM, honor killings, disproportionately low number of women in STEM, in government, among CEOs.
"... been using the facilities for their chosen gender for all that time. Hospitals and prisons have found a way to accommodate that, so have refuges"
That is because adult human males would have been challenged and told to leave women's shelters, changing rooms, and toilets. They would never have been allowed to compete against women in sports competitions. They would never be elected as women's officers or given women's scholarships. They would not shut down women's events and no-platform women speakers, and their ridiculous opinion that women talking about their biology is bigotry would not have been taken seriously.
"When thinking, "what will actually happen if self-ID comes in?", I came to the conclusion, "not an awful lot"."
If that is the conclusion you came to, I can only assume that you have not listened to women's concerns or if you have listened, not cared one bit.
I suggest that you read the above concerns again, slowly, then reflect on them until the various problematic consequences of not only self-ID but TWAW becomes clearer to you.